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Focusing Schlieren

Schlieren imaging is a popular technique for visualizing wavefront distortions or “phase objects” in transparent media, which can be related to such important physical parameters as temperature, density, and pressure. These parameters are critical in aerodynamic and fluid dynamic science and engineering. Schlieren imaging in general, which was developed in the 19th century,1 relies on forming an image with rays that pass by a sharp cutoff filter, which is arranged so that the ray intensity has a steep derivative along the edge of the filter. This arrangement allows small deviations in the path of the rays to produce a large change in the image intensity from the undeviated level. Classical schlieren systems use light collimated by optics such as mirrors or lenses, which effectively limits the area under test to the size of the optics. An important improvement on the original concept was the complementary-grid focusing schlieren system, originally described by R.A. Burton in 1949.2 Burton’s technique relied on the same edge filtering concept but instead of using collimated light, it images a background grid pattern onto a complementary opaque cutoff filter such that the filter edges lie along the edges of a light-dark edges of the pattern, while the target object is imaged in a different plane by the camera objective. The focusing approach generally allows for obtaining schlieren imaging through much larger areas without using the excessively large optics that make classical schlieren prohibitively expensive and difficult for large fields of view. Focusing schlieren approaches thus generally require less expensive equipment than traditional collimated-light schlieren systems and can more easily be scaled up to cover large areas, though previous approaches have still tended to be difficult to set up and align.

Diagram of a focusing schlieren system (high-speed strobe illumination)

The main challenge with focusing schlieren lies in the fact that the cutoff grid and the background grid have to be very precisely matched to get good sensitivity. This makes the system very sensitive to misalignment and aberrations in the optical system. In the late 2000s, NASA scientist Dr. Leonard Weinstein developed a new concept for generating the background grid wherein it was projected onto a screen through a a grid and optical system identical to the cutoff grid and camera optical system.3 This pair of identical optical trains ensures that the projected grid and cutoff grid experience the same optical aberrations. A similar approach was being investigated at the University of Witwatersrand in South Africa at around the same time.4 One of Spectabit Optics LLCs founding scientists, Dr. Drew L'Esperance, led an effort at MetroLaser Inc. to develop and test Dr. Weinstein's design concept, which led to the original Analog Schlierenscope, now a product available from Spectabit Optics LLC.

While at MetroLaser, Spectabit cofounders Dr. L'Esperance and Dr. Ben Buckner developed a new digital variant of Burton's focusing schlieren approach, Digital Focusing Schlieren, which retains one analog cutoff grid, allowing for high sensitivity, but replacing the background grid with a digitally generated image, either from a digital projector system or a digital display system. This patented approach allows software control to take over the task of aligning the grids and compensating for aberrations, resulting focusing schlieren system far less complex and more flexible than the analog system.

We have built high-speed versions of focusing schlieren with both high-speed cameras and high-speed strobe illumination, or a combination of them. Generally, our digital focusing system is capable of shorter exposures since we can directly back-light it rather than relying on projection, which is relatively inefficient in its use of the available light. It is possible to build back-lit analog systems, but the construction and alignment are extremely complex.

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